In a personal information device such as a cellular phone connected to a server via a network (hereinafter, referred to it as a mobile terminal), in order to prevent others from accessing to data stored in the device or the connected server, a system wherein the device is locked during a user is not using the device and user authentication is performed when the user starts to use the device has been employed.
However, if, for example, the user sets the mobile terminal to be locked at a timing that he or she closes it or does not perform a key operation over five minutes or more, and the user leaves the terminal opened on a desk carelessly, there arises a problem that other can make free use of the terminal continuously if the other steal the terminal four minutes after the user leaves it and starts illegally using it.
To dissolve this problem, it is effective to perform user authentication at a given timing for confirming whether an authorized user uses a mobile terminal even when someone is using the terminal continuously. Regarding this timing for performing the user authentication, a method for activating an operation of authentication at given intervals, for example at intervals of five minutes, has been proposed (for example, Patent Document 1).
Authentication processing requires, for example, inputting a password for password authentication or touching a fingerprint sensor for fingerprint authentication. This means that the user has to stop data processing having been performed just before the authentication. Accordingly, the usability for users decreases when the authentication is required many times.
On the other hand, in a case of a mobile terminal, a communication speed is fast in a place near to, for example, an access point or a base station, and is slow in a place far to the access point or the base station. Further, the communication speed is also slow in a crowded place such as a downtown since many users share the same communication band. In addition, the communication speed is slow in a time period when many users are participating in the communication because of congestion and, conversely, is fast in a time period when few peoples are communicating. Thus, the communication speed is changeable according to a place or a time period.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-55956